Optical stack for imaging directional backlights

Abstract

An imaging directional backlight apparatus including a waveguide, a light source array, for providing large area directed illumination from localized light sources. The waveguide may include a stepped structure, in which the steps may further include extraction features optically hidden to guided light, propagating in a first forward direction. Returning light propagating in a second backward direction may be refracted, diffracted, or reflected by the features to provide discrete illumination beams exiting from the top surface of the waveguide. Retarder stack arrangements are provided to reduce the display visibility to snoopers located in polar viewing regions of the display while achieving minimal reduction of head-on luminance. Further visibility of light reflections from automotive windscreens may be reduced.

The invention claimed is: 
     
       1. A backlight device comprising: a first array of plural light sources and a second array of plural light sources; a first waveguide comprising: a first input end extending in a first lateral direction along a first side of the first waveguide, and opposed first and second guide surfaces extending across the first waveguide from the first input end in a first longitudinal direction, the opposed first and second guide surfaces operable to guide light input at the first input end along the first waveguide, wherein the first waveguide is arranged to extract input light guided through the first waveguide to exit through the first guide surface in grazing directions with respect to the first guide surface; a second waveguide, comprising: a second input end extending in a second lateral direction along a second side of the second waveguide; and opposed third and fourth guide surfaces extending across the second waveguide from the second input end in a second longitudinal direction, the opposed third and fourth guide surfaces operable to guide light input at the second input end along the second waveguide, wherein the second waveguide is arranged to extract input light guided through the second waveguide to exit through the third guide surface in grazing directions with respect to the third guide surface, wherein the first and second longitudinal directions are in opposite directions, wherein the first array of plural light sources is disposed along the input end of the first waveguide and arranged to input input light into the first waveguide, wherein the second array of plural light sources is disposed along the input end of the second waveguide and arranged to input input light in to the second waveguide; a prismatic input layer arranged to receive the light extracted from the first and second waveguides in the grazing directions with respect to the first and third guide surfaces respectively and to direct the received light towards the normal to the plane of the prismatic layer; an output polariser arranged on an output side of the prismatic layer; an additional polariser arranged on the output side of the output polariser; and at least one retarder arranged between the additional polariser and the output polariser, wherein the light from the first and second waveguides is directed to provide first and second viewing windows, a control system arranged to control the first and second arrays of plural light sources, wherein the control system is arranged to provide switching between, in a first mode of operation, the first array of plural light sources being operated along the input end of the first waveguide, and a first applied voltage across the switchable liquid crystal retarder; and in a second mode of operation, the second array of plural light sources being operated along the input end of the second waveguide, and a second applied voltage across the switchable liquid crystal retarder, wherein the second applied voltage is different to the first applied voltage. 
     
     
       2. A backlight device according to  claim 1 , wherein the light from the respective first and second waveguides is directed to provide viewing windows having different widths. 
     
     
       3. A backlight device according to  claim 1 , further comprising a rear scattering reflector arranged behind the first and second waveguides. 
     
     
       4. A backlight device according to  claim 1 , further comprising an intermediate diffuser layer arranged between the first and second waveguides. 
     
     
       5. A backlight device according to  claim 1 , wherein at least one of the first and second guide surfaces of the first waveguide or the third and fourth guide surfaces of the second waveguide comprise microstructures that are arranged to extract input light guided through the first or second waveguides to exit through the first or third guide surfaces in grazing directions with respect to the first or third guide surfaces. 
     
     
       6. A backlight device according to  claim 1 , wherein the prismatic input layer comprises first input facets and second input facets, wherein the light extracted from one of the first and second waveguides is received by the first input facets and the light extracted from the other of the first and second waveguides is received by the second input facets. 
     
     
       7. A backlight device according to  claim 1 , wherein at least one retarder comprises a switchable liquid crystal retarder, and the display device further comprises a control system that is arranged to control the first and second arrays of plural light sources and is further arranged to control an applied voltage across the switchable liquid crystal retarder. 
     
     
       8. A backlight device according to  claim 1 , wherein in the second mode of operation, the first array of plural light sources is also operated along the input end of the first waveguide. 
     
     
       9. A backlight device according to  claim 1 , wherein the additional polariser is arranged on the input side of the input polariser and said at least one retarder is arranged between the additional polariser and the input polariser. 
     
     
       10. A backlight device according to  claim 9 , wherein the additional polariser is a reflective polariser. 
     
     
       11. A backlight device according to  claim 1 , wherein the additional polariser has an electric vector transmission direction that is parallel to the electric vector transmission of the input polariser in the case that the additional polariser is arranged on the input side of the input polariser or is parallel to the electric vector transmission of the output polariser in the case that the additional polariser is arranged on the output side of the input polariser. 
     
     
       12. A backlight device according to  claim 1 , wherein the additional polariser is arranged on the output side of the output polariser and said at least one retarder is arranged between the additional polariser and the output polariser. 
     
     
       13. A backlight device according to  claim 1 , wherein the at least one retarder comprises at least one switchable liquid crystal retarder and at least one correcting passive retarder. 
     
     
       14. A backlight device according to  claim 13 , wherein the at least one correcting passive retarder comprises a pair of retarders which have slow axes in a plane of the retarders that are crossed. 
     
     
       15. A backlight device according to  claim 13 , wherein the at least one correcting passive retarder comprises a retarder having a slow axis perpendicular to a plane of the retarder. 
     
     
       16. A backlight device according to  claim 13 , wherein the at least one correcting passive retarder comprises a retarder having a slow axis orientation with a component perpendicular to the plane of the retarder, and at least one component in a plane of the retarder. 
     
     
       17. A backlight device according to  claim 13 , wherein the at least one switchable liquid crystal retarder has an optical thickness between 500 nm and 1000 nm. 
     
     
       18. A backlight device according to  claim 17 , wherein the at least one correcting passive retarder has an optical thickness between 400 nm and 800 nm. 
     
     
       19. A backlight device according to  claim 1 , further comprising at least one further additional polariser and at least one further correcting passive retarder and at least one further switchable liquid crystal retarder layer arranged between the at least one further additional polariser and the input polariser in the case that the further additional polariser is arranged on the input side of the input polariser or between the further additional polariser and the output polariser in the case that the further additional polariser is arranged on the output side of the input polariser.